There have been proposed various types of pneumatic nail drivers as a kind of pneumatic impact tool driven and controlled by compressed air containing lubricating oil. One of the known nail driver has a grip formed as a part of the housing, for an easier handling and transportation by the operator. The housing contains a piston-cylinder mechanism, head valve for starting or stopping the supply of compressed air to the impact piston-cylinder mechanism, and a manually operable trigger valve for controlling the head valve. More specifically, the trigger valve is switched by a manual operation of a trigger lever, so that the head valve is switched to the position for supplying the compressed air. In consequence, the upper chamber of the impact cylinder formed at the top dead center of the impact piston is communicated with a compressed air source through an air hose, so that the compressed air is instantaneously supplied from a compressed air chamber into the upper chamber in the impact cylinder, so that the pressure of the compressed air acts on the impact piston to instantaneously drive the impact piston to the bottom dead center of the impact piston thereby to drive the nail into an object.
Another type of known pneumatic nail driver has a rod-shaped driver adapted to reciprocatingly move in a nose. A trigger safety arm mechanically connected to the trigger lever is movable reciprocatingly in the longitudinal direction of the nose. As the trigger lever is manually actuated while pressing the contact surface of the trigger safety arm against the object surface, the impact piston is moved to drive the nail into the object surface.
Each of such known pneumatic nail drivers incorporates a head valve piston adapted to move between the top dead center and the bottom dead center by the difference of the total pressure of air acting on the upper and lower surfaces, and a valve spring adapted for assisting the resetting movement of the head valve piston. The valve spring usually keeps the head valve piston stationary at the bottom dead center, so as to disconnect the upper chamber of the impact cylinder from the compressed air storage chamber. Therefore, when there is no time lag of the application of compressed air on the upper surface of the head valve piston in relation to the application of compressed air to the lower surface of the same, the compressed air is not allowed to flow into the upper chamber of the impact cylinder even at the instant at which the compressed air is supplied to the compressed air storage chamber, so that the accidental discharge of the impact piston (referred to as initial discharge of the impact piston, hereinunder) is avoided. However, in the pneumatic nail driver to which the invention pertains, a part of the lower surface of the head valve piston is directly exposed to the compressed air storage chamber, while, the compressed air supplied from the compressed air storage chamber via a control air passage including a trigger valve generating a throttling effect is applied to the upper surface of the head valve piston. As a result of this arrangement, at the instant of supply of the compressed air to the compressed air storage chamber, the compressed air acts on the lower side of the head valve piston earlier than on the upper side of the same. In consequence, the force of the compressed air acting on the lower side of the head valve piston drives the latter toward the top dead center, overcoming the force of the valve spring, so that the upper chamber of the impact cylinder is brought into communication with the compressed air storage chamber. In consequence, the initial discharge of the impact piston is caused undesirably at the instant at which the compressed air storage chamber is connected to the compressed air source.
In this case, the aforementioned valve spring performs no substantial function.
Further, as stated before as to function of the valve spring, the lubricating oil is atomized and contained by the compressed air. This lubricating oil increases its viscosity when the nail driver is used at a low temperature, so as to hinder the correct operation of the valve spring in the head valve. Thus, it is often experienced that, at the time of restarting of the nail driver after a suspension of the use, the valve spring has not completely reset the head valve piston, so that the latter is positioned intermediate between the top and bottom dead centers to maintain the upper chamber of the impact cylinder in communication with the compressed air storage chamber. The undesirable initial discharge of the impact piston takes place also for this reason.
Generally, when the pneumatic nail driver is connected to a compressed air source, the operator is not ready for the work, and nose of the nail driver is often directed toward a part of the personal body, particularly the foot. If the initial discharge of the impact piston takes place in such a state, the operator or any person in his vicinity will be injured accidentally by the nail discharged from the nail driver.
The trigger safety arm incorporated in the second type of known nail driver functions as a safety mechanism which prevents, in connection with the manual operation of the trigger lever, the mis-shooting during the nail driving work. Thus, this mechanism is quite invalid for preventing the mis-discharge which may occur when the compressed air chamber is connected to the compressed air source in the preparation of the work.
When the operator who has completed the work at one place moves to the near another place, he holds the grip of the nail driver by a single hand and, moreover, pulling the trigger lever in order to disolve the unbalance of weight of the nail driver, without disconnecting the air hose leading from the compressed air source, from the nail driver. Therefore, if the contact surface of the trigger safety arm is happened to contact something during transportation of the nail driver and the trigger safety arm is caused to move in the longitudinal direction of the nose, the mis-discharge will take place possibly resulting in an injury of the operator's foot.
It is often necessary that, due to relationship between one and another work places, the air hose leading from the compressed air source is temporarily disconnected from the nail driver. The aforementioned valve spring of the head valve and the trigger safety arm are invalid or insufficient for completely eliminating the mis-discharge which may take place when the hose is connected again to the nail driver.
Further, the trigger safety arm often fails to be reset to the operative position after the stop of the nail driving work. If the operator pulls the trigger lever in such a state for transportation of the nail driver, the mis-discharge will take place to injure the operator.